Xtrack version 0.42.0

Changes:

• Introduce thick Solenoid beam element.
• Introduce stability diagram calculation.
• Introduce BeamPositionMonitor, BeamSizeMonitor, BeamProfileMonitor.
• Support thick elements in line definition from xsuite sequence.
• Support element-by-element particles monitor in collective mode.
• Introduce mode="thick" in slicing strategies.
• Introduce cross-platform atomic add headers/atomicadd.h.

Full Changelog: v0.41.2...v0.42.0